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Abstract:

The invention relates to a method for preparing potable water from crude
water containing trace species contaminants. The method includes the
steps of separating iron compounds and optionally other compounds from
the crude water, contacting the water with a ferrous material,
co-precipitating trace species upon aeration, and recovering drinking
water.

Claims:

1-7. (canceled)

8. A method for preparing potable water from crude water containing trace
species contaminants, said method comprising the steps of: i. separating
iron compounds and optionally other compounds from the crude water; ii.
contacting the water with an iron-containing material under
sub-atmospheric oxygen partial pressure such as to enrich the water with
Fe(II) compounds; iii. co-precipitating at least a part of the trace
species by treating the iron-enriched water under oxidizing conditions in
an aerator; and iv. recovering potable water by separation of the
precipitate.

9. The method according to claim 8, wherein the separation in step i.) is
effected in a sand filter.

10. The method according claim 8, wherein the iron-containing material is
iron ore or metallic iron, including iron particles, iron filings or
swarfs, or any other natural iron-containing material presenting an
extended surface area.

11. The method according to claim 8, wherein in step ii.) said contacting
is effected by pumping the water into a closed container to a bed of said
iron-containing material.

12. The method according claim 8, wherein in step iii.) said treating
under oxidizing conditions in an aerator is achieved by leading the water
enriched with Fe(II) to the top of an aerator, optionally from an
iron-containing material being enclosed in a container mounted above the
aerator, said aerator comprising a plate or one or more pipes with holes
or slots for forming drops by flow of the water through the holes or
slots at the initiation of the treatment process, and means for causing
division of the drops by contact therewith, said means being arranged
below said plate or pipe(s), wherein the means for causing division of
the drops comprise a plurality of tubular elements in the form of pipes
having reticulate pipe walls, said tubular elements being placed in
horizontal layers of several parallel tubular elements stacked in such a
way that the longitudinal axes of the tubular elements in one layer are
angularly displaced in relation to the longitudinal axes of the tubular
elements in the one or more adjacent layers; and letting the water pass
through said aerator to the bottom thereof by the force of gravity.

13. The method according to claim 8, wherein in step iv.) the precipitate
is separated from the drinking water by settlement in a collection
container, optionally followed by further separation by treatment of the
water in a sand filter.

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to a method for preparing potable
water from crude water containing trace species contaminants.

BACKGROUND

[0002] The contamination of groundwater with contaminant substances
constitutes a major problem throughout the world. Where ever
industrialised farming is carried out, the occurrence of pesticides and
their breakdown products in groundwater is commonplace, but also the
natural constitution of the underground itself may give rise to serious
cases of contamination.

[0003] Thus, the presence of excess amounts of arsenic in more than 3
million groundwater wells of the world is linked to an increased risk of
cancer and a range of other diseases and health problems in the affected
areas, among which Bangladesh is often mentioned as a case of extreme
affliction.

[0004] Due to the adverse health effects of arsenic, the WHO has lowered
the recommended limit for arsenic in drinking water to 10 μg/L and in
many industrialised countries the limit is now set at 5 μg/L. However,
this has resulted in a large number of waterworks failing to comply with
this limit using existing methods, and they have either had to close down
or to invest in costly equipment for purification. It has been found a
difficult task to reduce the content of arsenic from a frequently
encountered level of 20-35 μg/l down to below 5 g/l at a reasonable
cost.

[0005] Generally speaking, the problem is particularly pronounced in
waterworks receiving groundwater with a low content of iron. In
waterworks endowed with water rich in iron compounds this is sometimes
less so, since arsenic under certain circumstances may be co-precipitated
with oxidized iron compounds, when the water is treated in a conventional
way by oxidation, typically aeration, until iron precipitates in sand
filters or precipitation basins. However, it is not possible to remove
arsenic by conventional oxidation of water, if the iron content of the
water is not sufficient to ensure the desired co-precipitation of arsenic
and other contaminants, including pesticides.

[0006] Remarkably, though, it has been observed in recent years that even
some crude water supplies showing substantial concentrations of iron
compounds are associated with contamination by arsenic and other trace
species, which due to special circumstances do not--or only
insufficiently--co-precipitate with iron upon oxidation.

[0007] DE 197 45 664 A1 discloses a method for treating arsenic-containing
water, where the water flows through a reactor filled with an
iron-containing granulate, said granulate being produced by mixing sand
and iron powder and subsequent firing under exclusion of oxygen. in the
reactor, the iron is oxidized by the oxygen dissolved in water generating
Fe(III) ions, said ions together with As forming poorly soluble iron
arsenate. Excess Fe(III) ions are precipitated as iron hydroxide binding
As by adsorption. Thus, As binds to the granulate, wherefrom it has to be
removed at suitable intervals. When precipitating Fe(III) compounds, the
granulate uses agglomerates comparatively quickly and has to be exchanged
frequently. The manufacture of the granulate requires work and energy.
Moreover, the method requires the supply of additional oxygen to the
reactor prior to treatment, if the treated groundwater is low in oxygen.
in conclusion, the known method is work-intensive, complicated and
expensive.

[0008] U.S. Pat. No. 5,951,869 describes a reactor, where water is treated
with iron while simultaneously supplying oxygen. The treatment takes
place in a fluid bed with iron particles as the source of iron. The use
of a fluid bed, though, is an expensive and cumbersome enterprise.

[0009] The above-mentioned methods share the common feature that the iron
treatment takes place concomitant with aeration or requires that the
water has a suitable content of oxygen from the very start. Accordingly,
there is an increased risk that the system is clogged by the precipitated
oxidized iron compounds.

[0010] US 2009/0020482 marks a great step forward in the development of
methods for removing contaminant trace species. Here, the water to be
treated is contacted with an iron-containing material prior to aeration
in order to increase the iron content of the water and thus improve
co-precipitation of contaminants upon oxidation.

[0011] However, as alluded to in the above, under certain circumstances
trace species contaminants do not co-precipitate with iron upon aeration,
when a substantial level of iron is inherently present in the water to be
treated. In that case, it has been found, co-precipitation of said
contaminants cannot be brought about to a satisfactory degree, either, by
contacting the crude water with an iron-containing material prior to
aeration.

[0012] Accordingly, it has been natural to conclude that the concept of
iron-enrichment and ensuing aeration is not practicable for removing
arsenic and other undesired trace species from crude water being already
rich in iron and all the same showing poor co-precipitation of the trace
species upon aeration.

SUMMARY

[0013] In view of the above, an object of the present invention is to
provide a method for production of drinking water from crude water
containing: trace species contaminants, wherein an effective and
efficient removal of contaminants to a satisfactory level is attained,
also when starting from iron-rich crude water from which the trace
species contaminants do not co-precipitate sufficiently following
aeration of the water. The method should furthermore be affordable,
simple and environmentally friendly.

[0014] To meet this object, according to the invention a method is
provided for producing drinking water from crude water containing trace
species contaminants, said method comprising the steps of separating iron
compounds and optionally other compounds from the crude water; contacting
the water with an iron-containing material under sub-atmospheric oxygen
partial pressure such as to enrich the water with Fe(II) compounds;
co-precipitating at least a part of the trace species by treating the
iron-enriched water under oxidizing conditions in an aerator; and
recovering potable water by separation of the precipitate.

[0015] It has surprisingly been found by the inventors that the
counterintuitive procedure of first clearing away, then adding iron is
very effective in achieving a consistent removal of trace species
contaminants, irrespective of the composition of the crude water to be
treated.

[0016] With the finding of the inventors, an inexpensive and simple method
is provided, which method requires only a small consumption of energy and
no extraneous chemicals besides the iron-containing material, and wherein
the purifying capacity of said material is turned fully to account.

[0017] Preferably, the initial separation of iron compounds and optionally
other compounds from the crude water is effected in a sand filter.
Additionally or alternatively, use of other filter types as well as
settlement in a collection container may come into consideration.

[0018] According to a preferred embodiment of the invention, the
iron-containing material is iron ore or metallic iron, including iron
particles, iron filings or swarfs, or any other natural iron-containing
material presenting an extended surface area. The Fe(II) compounds may be
added to the water in a simple and reliable manner at acceptable cost by
making use of these. Filings and swarfs are available as cheap waste
products in the form of calcinated waste iron from cutting machines.

[0019] Preferably, the crude water is contacted with the iron-containing
material in a closed container by pumping the water onto a bed of said
iron-containing material. Throughout the present text, a "closed
container" is to be understood as a container provided with openings for
inlet and outlet of the water to be treated but with substantially no
further openings during performance of the method according to the
invention. By making use of a closed container, the observance of a
sub-atmospheric oxygen partial pressure is facilitated, so that premature
precipitation of Fe(III) compounds is minimized After contacting with the
iron-containing material, the water may leave the bottom of the bed by
means of suitable openings.

[0020] Advantageously, a layer of green rust is maintained on the surface
of the iron-containing material.

[0021] Preferentially, the water is treated under oxidizing conditions by
leading the water enriched with Fe(II) to the top of an aerator,
optionally from a bed of iron-containing material mounted above the
aerator, said aerator comprising a plate or one or more pipes with holes
or slots for forming drops by flow of the water through the holes or
slots at the initiation of the treatment process, and means arranged
below said plate or pipe(s) for causing division of the drops by contact
therewith, wherein the means for causing division of the drops comprise a
plurality of tubular elements in the form of pipes having reticulate pipe
walls, said tubular elements being placed in horizontal layers of several
parallel tubular elements stacked in such a way that the longitudinal
axes of the tubular elements in one layer are angularly displaced in
relation to the longitudinal axes of the tubular elements in the one or
more adjacent layers; and letting the water pass through said apparatus
to the bottom thereof by the force of gravity.

[0022] Preferably, the aerator is fit up so that the longitudinal axes of
the tubular elements in one layer are angularly displaced in relation to
the longitudinal axes of the tubular elements in the one or more adjacent
layers by an angle of approximately 90° C. In this way, good
overall conditions for drop divisions to occur within the aerator are
generated.

[0023] According to a preferred embodiment, the precipitate formed by
treatment of the water in the aerator is separated from the drinking
water by settlement in a collection container. Thereafter, the water may,
if required, be led to one or more filters, e.g. sand filters, for
further purification. It may, however, be relevant to return the water
one or more times after precipitation and separation of the iron
compounds for renewed contact with the iron-containing material, so that
the content of trace species may be brought even further down.
Alternatively, the water may be returned from the bottom of the aerator
to its top with a view to enhanced aeration. Furthermore, air, optionally
enriched in oxygen, may be led by passive or active flow in a vent pipe
to the part of the aerator containing the tubular elements. In this
manner, the degree of oxidation achieved in the aerator may be further
regulated. The active supply of oxygen to the aerator could be used as an
alternative to return of water from the bottom to the top of the aerator.

[0024] In an alternative embodiment, the precipitate is separated from the
drinking water by direct dripping of water treated under oxidizing
conditions onto an open sand filter without any intermediate settlement
in a collection container, the precipitate being deposited on or near the
upper surface of the sand filter. By ensuring a thorough aeration, a
fully satisfactory flocculation of undesired compounds may in some
instances be achieved, resulting in the formation of floc, which
accumulates on the surface of the sand filter without infiltrating this,
so that it can be easily removed.

[0025] Preferably, the co-precipitated trace species comprise arsenic
and/or pesticides and/or non-volatile organic carbon (NVOC) such as
humus. However, also other trace species such as chromium, mercury, MTBE
(methyl t-butyl ether), and a range of non-pesticide chlorinated
hydrocarbons may be co-precipitated.

BRIEF DESCRIPTION OF THE DRAWING

[0026] In the following, a preferred embodiment of the invention will be
illustrated by reference to the non-limiting figure.

[0027]FIG. 1 illustrates an embodiment of a plant for carrying out the
method according to the invention.

DETAILED DESCRIPTION

[0028] Referring now to the figure, the main features of the illustrated
plant are referenced by numbers as follows: 1 is a separator unit for
separation of iron compounds and optionally other compounds from crude
water, which is subsequently pumped to the top of the plant to a drip
tray 2; 3 is a bed of iron swarfs arranged in a perforated plastic tray
4; 5 is an aeration chamber of an aerator; 6 is a collection container; 7
is a pump for leading the water to a sand filter 8; 9 is an outlet for
pure drinking water; 10 is a pump for pumping treated water from the
collection container 6 to the top of the plant for repeated treatment.

[0029] An overall description of a preferred embodiment of the method
according to the invention will now be given.

[0030] An amount of crude water rich in iron is received in the separator
unit 1. The separator unit in this embodiment is constituted by a closed,
rapid sand filter made up of coarse sand and showing a high flow rate. It
is regularly cleaned by backwashing. Alternatively, the sand filter might
have been of the slow type relying on biological processes for its
functioning and depending on the formation of a gelatinous layer of
living organisms known as a "Schmutzdecke" in the uppermost few
millimetres of the fine sand layer of the filter. In that case, the
filter would have been rejuvenated by scraping off the top layer of the
filter to expose a new layer of fresh sand.

[0031] Without wishing to be bound by a specific theory, it is believed
that the very remarkable effect, which is conferred on the overall
process by the treatment in the separator unit, is due to the fact that
the water is freed from iron compounds in an inactive state, which are
not able to bind trace species contaminants. If the crude water is
saturated with such inactive compounds when contacted with the
iron-containing material, the ferrous material, which would otherwise be
released from the iron-containing material and precipitate together with
contaminant trace species, is inhibited in exerting its function.

[0032] A possible explanation for the occasional inactivity of iron
contained in the crude water might be its association with humic or other
organic substances; also, the iron may be in the form of particles, which
are shielded by bacterial encrustations.

[0033] From the separator unit 1 the water is pumped to the drip tray 2,
wherefrom it is uniformly distributed across a bed of iron swarfs 3
approximately 10 cm thick, said bed being arranged in a perforated
plastic tray 4. The dimensions of the bed of swarfs is determined so that
the necessary uptake of iron compounds is assured for effective binding
and co-precipitation of present arsenic, pesticides and other harmful
trace species. The iron swarfs are available as a waste product of
machining and have been calcinated prior to their use to remove residual
cutting oil.

[0034] In order to maintain a layer of green rust on the surface of the
iron swarfs, the oxygen concentration in the crude water at the time of
contacting the iron swarfs is kept at a stable level close to 1 mg/L,
while the corresponding pH of the water is also monitored and kept close
to a value of 6.5.

[0035] It is assumed that the green layer formed in the present case is
green rust of the kind, which incorporates carbonate ions. When corroding
in the presence of an aqueous medium, iron starts by dissolving, and then
reacts with the aqueous medium to form ferrous hydroxide Fe(OH)2,
where iron is divalent (FeII). Subsequently, this compound is
transformed into the products of green colour, called "green rusts",
which is stable only at very low levels of oxygen. These green rusts at
the same time contains divalent (FeII) and trivalent (FeIII)
iron. The composition of green rust formed in the presence of carbonate
is [FeII4FeIII2(OH)12]2+
[CO32H2O]2-.

[0036] In groundwater arsenic is present as arsenite
(H2AsIIIO3-) and/or arsenate (HAsvO42-).
Ions of arsenate adsorb to groups of --OH2.sup.+ in the layer of
green rust, while ions of arsenite apparently are not able to do so
before being oxidized themselves to arsenate.

[0037] However, the green rust also contains the carbonate anion
CO32- and there is evidence to suggest that said carbonate ions
may be exchanged by arsenite, which is then catalytically converted into
arsenate by the content of FeIII in the layer of green rust. This
may explain the very effective removal of arsenic found when making use
of green rust.

[0038] The iron-oxidizing, chemolithotropic bacteria Gallionella feruginea
is also worth keeping on the iron swarfs. It has proven very useful in
the removal of contaminant trace species as it precipitates Fe oxide in
the form of ferrihydrite, which is a nanoporous hydrous ferric
oxyhydroxide mineral presenting a large surface area of several hundred
square meters per gram. In addition to its high ratio of surface area to
volume, ferrihydrite also has a high density of local defects, such as
dangling bonds and vacancies, which all confer to it a high ability to
adsorb many environmentally important chemical species, including
arsenic.

[0039] Also with a view to the workings of the green rust and the
iron-oxidizing bacteria as described in the above, it is of great
significance that undesirable iron compounds is separated from the crude
water at the beginning of its treatment. Leaky crude water pipings as
well as aquifers from strata rich in pyrite may give rise to premature
biological oxidation of the iron present in the crude water, resulting in
the development of an ochreous, slimy layer on the iron-containing
material employed according to the invention and thus impeding its
function.

[0040] Moreover, the initial separation treatment may have a beneficial
effect in retaining excess amounts of CaCO3, which would otherwise
deposit as a passivation layer on the iron-containing material in case of
a low content of CO2 in the crude water.

[0041] The tray 4 is provided with a plurality of holes, e.g. having a
diameter of 3-4 mm The water eventually arrives as drops in the top of
the aeration chamber 5 for treatment of water. By the force of gravity
said drops fall and impinge on a multitude of alternating layers of
tubular elements, mutually displaced by 90°, so that the drops are
divided into droplets. The formation of droplets results in a
substantially larger drop surface area relative to drop volume, so that
enhanced enrichment with oxygen can take place. The height of the stack
of layers of tubular elements is adjusted so that the initial drops are
divided at least 50-60 times and preferably 60-80 times when falling
through the aeration unit, in which case a satisfactory oxygen saturation
of up to 95% is assured. Alternatively, the water might have been aerated
in a conventional device such as a splasher, a drip-type sheet, a cascade
aerator or by blowing in air or oxygen.

[0042] The aerated droplets of water is directed to the collection
container 6, where oxidized iron compounds settle together with
co-precipitated trace species contaminants. The settled material may be
removed from the collection container as necessary by light flushing. The
water is fed to the sand filter 8 by means of the pump 7 to effect
further precipitation of iron and trace species, and finally drinking
water is taken out from the outlet 9. In many other cases, however,
separation in the collection container would have been perfectly
sufficient, so that the final sand filtration might have been dispensed
with.

[0043] The concentration of arsenic and other trace species in the final
drinking water product is monitored on a regular basis and when
increasing towards the stipulated limit, the bed of iron swarfs 3 is
replaced as an integral, closed unit together with its underlying plastic
tray 4 and overlying drip tray 2. Accordingly, the method may be
performed by persons without any specialised training and is usable in
developing countries as well as in industrialised countries.

EXAMPLE

[0044] A plant for performing the method according to the invention is
installed at a waterworks receiving crude water showing a high content of
arsenic (>20 μg/L) and a high content of iron (>1 mg/L), which
is unable to co-precipitate arsenic, i.e. presenting quite difficult
conditions for satisfactory removal of arsenic.

[0045] The content of oxygen in the water when contacting iron swarfs is
kept below 1 mg/L. On the iron swarfs a layer of green rust is developed
and maintained. During the development of said layer, a series of
analyses is made of the content of iron in the water following passage
through the initial separator unit as well as the iron swarfs. First a
dramatic decline in iron to about 0.4 mg/L is seen, whereupon the level
rises again during the course of two months to reach a level of more than
1 mg/L again. Now, however, the iron in the water is of another type,
which is able to co-precipitate arsenic. This is reflected by the
measured content of arsenic in the water following sedimentation and
filtration in a sand filter; said content drops from the initial level of
more than 20 μg/L to a level of less than the stipulated limit value
of 5 μg/L.